1. Field of the Invention
The present invention relates to a vehicle stability control system, and more specifically, to an integrated system utilizing steering and braking controls.
2. Description of the Related Art
Vehicle braking systems are known to include antilock braking (ABS), traction control (TC), and vehicle stability control (VSC). ABS systems monitor the wheel rotational behavior and selectively apply and relieve hydraulic brake fluid pressure in the respective vehicle brakes via electronically controlled isolation valves and dump valves for maintaining wheel speed within a selected slip range while maintaining optimum braking forces. Such controlled braking may be applied on an individual vehicle wheel, a combination of vehicle wheels, or to all vehicle wheels.
In traction (TC) control systems, additional valves are added to an existing ABS system to control wheel speeds during vehicle acceleration. Excessive wheel speed during vehicle acceleration may produce wheel slip and loss of traction. When this condition is sensed, braking pressure is automatically supplied to the vehicle brakes of the slipping wheels to reduce slippage and regain uniform traction between the wheels.
A vehicle stability control (VSC) braking system is used to improve the stability of a vehicle by counteracting forces otherwise leading to the instability through the application of selective brake actuation. Such instability can be caused during vehicle motion such as cornering. Each of the above systems utilize braking as a means to improve vehicle stability for a given vehicle instability behavior.
Control assist steering systems have been developed to aid a driver in steering efforts for controlling the stability of a vehicle. For example, if a vehicle instability is occurring from an oversteer or understeer condition, sensors detect the instability condition of the vehicle. A controller within the control assist steering system generates a steering torque output in the direction required for correcting the instability of the vehicle. This control action is performed prior to the driver identifying and reacting to the instability condition.
Some systems have utilized both the VSC system through braking and the control assist steering system to cooperatively correct the instability condition of the vehicle and shorten the stopping distance of the vehicle as opposed to using either the VSC or control assist steering system individually.
One such instability condition in which VSC and control assist steering may work in cooperation with one another is when a vehicle is driving along a split-mu surface. That is, the road has a high-mu surface (e.g., dry surface) under the wheels on one side of the vehicle and a low-mu surface (e.g., ice) under the wheels on the opposite side of the vehicle. Performing a vehicle stability control operation would exert a high braking force on the front vehicle wheel in contact with the high-mu surface and a low braking force on the remaining wheels. The vehicle would tend to yaw about the center axis of the vehicle in a direction of the wheel with the high braking force. For example, a high braking force on the front-left wheel (high-mu surface) would cause the vehicle to yaw counterclockwise. The control assist steering system would generate a steering torque to create a yaw force in the opposing direction (clockwise). If however, the driver of the vehicle does not allow the control assist steering system to directionally correct the steering wheel angle, and if the high braking force is maintained on the front-left wheel, the vehicle may have a tendency to spin out.